Excerpt:"Now let's look at some examples of feedback carburetor
problems, to help you see how this would work. All these examples
have really existed in reality or at least as close to reality
as I can remember.
Example #1:
This '85 Mazda failed a smog test, but had no other complaints
from the customer. The readings from the emissions report
showed about 4% CO at the 2500 RPM part of the two speed idle
test. All other readings were normal.
(Well, OK, the HC was also a bit high at 2500, but you could
see it was just because of the CO.) So what are we to think?
The first test was to see if the O2 sensor knew the system
was running rich.
Sure enough, the O2 sensor showed about 0.9 v. So we have
a real condition that is rich, and the computer knows the
condition is rich. Shouldn't the computer command be to lean
out the condition? How would we test this? Yes, you knew.
We measure the command at the mixture control solenoid. Since
this is a normally rich carburetor, and the solenoid leans
out the mixture when the computer grounds the circuit, we
can just measure the voltage and see the average to tell what
the command is.
In this case, the voltage was real low, I think between
1and 2 volts. This tells us the computer is trying to ground
the circuit a lot, to lean it out. So is this a computer problem?
No. Do we need to ground the solenoid ourselves to see if
the system can go lean? We could try that. But the computer
is already doing that. We could unplug the solenoid to see
if not grounding it makes any difference. (Ya, best to do
this when the engine is off, to avoid voltage spikes say some
people.)
But this doesn't make any difference. So is the carburetor
responding to the computer command? No. So what would you
do? Yes again. There has got to be something in the computer
preventing the command from working. And in this case the
solenoid part that blocks off the fuel to make it lean just
isn't working. It seems stuck.
You can ground the solenoid and ..."
The article contains these sections:
- Example #1
- Example #2
- Example #3
- Example #4
